In a mobile communication system, when a transmitting side transmits a packet, a receiving side may inform the transmitting side whether or not the packet has been successfully received. If the reception of the packet is successful, the receiving side transmits an acknowledgement (ACK) signal to cause the transmitting side to transmit a new packet. If the reception of the packet fails, the receiving side transmits a negative acknowledgement (NAK) signal to cause the transmitting side to re-transmit the packet. Such a process is called an automatic repeat request (ARQ) operation.
Meanwhile, hybrid ARQ (HARQ), which is a combination of the ARQ operation and a channel coding scheme, has been proposed to lower an error rate by combining a re-transmitted packet with a previously received packet and to improve the overall efficiency of the system. In order to increase throughput of the system, HARQ demands a rapid ACK/NAK response from the receiving side compared with a conventional ARQ operation. Therefore, the ACK/NAK in HARQ is transmitted by a physical channel signaling method. A method for achieving HARQ may be broadly classified into two schemes.
The first scheme is chase combining (CC). Chase combining serves to transmit a packet during re-transmission using the same modulation method and the same coding rate as a modulation method and a coding rate used to transmit a previous packet. The second scheme is incremental redundancy (IR) which serves to transmit a packet during re-transmission using a different modulation method and a different coding rate from a modulation method and a coding rate used to transmit a previous packet. When using the second scheme, the receiving side can additionally obtain a coding diversity gain, thereby increasing the processing capacity of the system.